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This post is an explanation of the calculation/detection mechanics involved in very weak signal reception such as that of the WSPR signals.The above diagram shows the average noise floor of my 140 m Beverage antenna for 15 days in December 2018. In June 2019, as described here:

the antenna was modified, brought to 160 m and terminations optimised, so that an average of 6 dB noise improvement on Noise Floor was obtained. Thus I could expect to have an average antenna Noise Floor of - 111 dBm @ 7 MHz.

Keeping in mind the specified Noise Figures shown in the previous post, measurement of the terminated noise floors of my own RSP-1A and RSPduo yelded the following results @ 1800 Hz (LSB) detection bandwidth:

The reference bandwith for the WSPR, e.g. the reference at which the S/N ratio is measured, is 2500 Hz, thus we may have a measurement error of approx. 1.5 dB by using the reference value of 1800 Hz. This is not so important, considering that the final autocorrelated detection bandwidth of the WSPR signal is 8 Hz. The worst case S/N value of WSPR signal is -28 dB, that is the WSPR signal that will be detected with an S/N value of -28 dB or better, referred to the bandwidth of 2500 Hz.

Considering that our antenna noise floor is -111 dBm, the above calculated WSPR signals will be far buried into the noise of the SDRuno dial and therefore invisible. This is why, for WSPR operation, the frequency precision and stabilty of the SDR Processors are essential and indispensable for successful reception.

All the above we have seen is just an example based on my Beverage antenna performance data. If the antenna Noise Floor was, for example, -125 dBm, then with an RSPduo one could detect a WSPR signal at a level of -139 dBm.

In any case, as shown in the thread about "Getting ready for Diversity reception", with RSPduo in Diversity operation, I have uploaded an example of full copy of an SSB signal at - 117 dbm with the same Beverage plus an auxiliary 60 m long wire antenna.

I have some links to reference material on the concept of Noise Figure (NF) and how atmospheric and man-made noise is measured. I hope readers find it useful for understanding some of the issues being discussed here.

There are many tutorial articles and books that discuss the concept of Noise Figure and how it is used. The following article from from Wikipedia is quite informative and I suggest readers unfamiliar with this subject take a look at this article. https://en.wikipedia.org/wiki/Noise_figure

The term Antenna Noise Figure (Fa) is a related concept and is very useful when discussing noise that is received on any antenna. Natural noise (galactic, lightning, atmospheric etc.) and man-made noise can significantly affect reception. Characterizing this noise and measuring the level at different locations, time and in different seasons has been a subject of study for many years. An updated version of ITU Publication 372 which discusses this topic is freely available for download here: https://www.itu.int/rec/R-REC-P.372-13-201609-I/en

Bill

Last edited by SDRguy22 on Fri Jul 26, 2019 3:41 pm, edited 1 time in total.

I indeed welcome your tractation and I am glad to have found a Forum member founding his Antenna Noise analysis on the notorious ITU publication 372. I really would like you to examine my one year old work and Antenna measurement cycle based on the mentioned publication here:

These two studies, based on the important ITU Recommendation ITU-R P.372-13, are the foundation for modern HF receiving system performance assessment, based on field measurements and data analyses, which yeld significant and immediate practical results.

With the cited modification the Beverage antenna Noise Floor increased less than at the lower frequencies, but still an improvement of 3 dB was obtained, bringing the expected average Noise Flor at - 115 dBm. Terminated Noise Floor at 14 MHz of my own RSP-1A and RSPduo yelded the following results @ 1800 Hz (LSB) detection bandwidth:

We repeat here: reference bandwith for the WSPR, e.g. the reference at which the S/N ratio is measured, is 2500 Hz, thus we may have a measurement error of approx. 1.5 dB by using the reference value of 1800 Hz. This is not so important, considering that the final autocorrelated detection bandwidth of the WSPR signal is 8 Hz. The worst case S/N value of WSPR signal is -28 dB, that is the WSPR signal that will be detected with an S/N value of -28 dB or better, referred to the bandwidth of 2500 Hz.

We can conclude that long distance, WSPR reception of QRP signals has better chances of success in the 20 m band, because, while Noise Floor level is generally lower than at 40 m, the antenna dimensions are halved and it is easier for the average operator to approach this new mode. Additionally the Noise Figure advantage of the RSPduo over the RSP-1A on 20 m amounts to 0.8 dB only, so there is practically no difference between the two processors' performance.